error.c: redefined backtrace

* error.c (rb_get_backtrace): honor redefined Exception#backtrace
  method.  [ruby-core:78097] [Bug #12925]

* eval.c (setup_exception): rescue exceptions during backtrace
  setup.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56767 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_exception.rb

@@ -965,4 +965,38 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
       }
     end;
   end
+
+  def test_redefined_backtrace
+    assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}")
+    begin;
+      $exc = nil
+
+      class Exception
+        undef backtrace
+        def backtrace
+          $exc = self
+        end
+      end
+
+      e = assert_raise(RuntimeError) {
+        raise RuntimeError, "hello"
+      }
+      assert_same(e, $exc)
+    end;
+  end
+
+  def test_wrong_backtrace
+    assert_separately([], "#{<<-"begin;"}\n#{<<-"end;"}")
+    begin;
+      class Exception
+        undef backtrace
+        def backtrace(a)
+        end
+      end
+
+      assert_raise(RuntimeError) {
+        raise RuntimeError, "hello"
+      }
+    end;
+  end
 end